1. Field of the Invention
The present invention relates generally to automatic detection of structures characterized by a change in brightness level compared with its surrounding when the information is carried by a video signal, and more particularly to the automatic detection of the left ventricular contour.
2. Description of the Prior Art
Dynamic measurements of the size and shape of the left ventricular cavity and their correlation with simultaneously occurring pressure and flow events are of great importance in cardiovascular research. Many methods are known for measuring cardiac chamber dimensions and volume in animals and man. However, angiocardiography has proven with time to be the most readily available and reliable method for these purposes. By injecting a suitable radiopaque liquid (usually Renovist) into the left ventricle, a silhouette of the left ventricle can be generated at the face of an image intensifier with X-rays from an X-ray tube underneath the patient. When using the negative of conventional film, the X-ray picture shows the left ventricle as a light structure against a dark background. In the border region between the ventricular silhouette and its background the so-called ventricular contour can be visualized. Certain mathematical methods enable the left ventricular volume to be estimated from this contour data.
The ability to obtain radiographic information concerning chamber size and shape for clinical or investigative use within reasonable periods of time is limited. Presently, the acquisition of such data usually requires the manual tracing of heart outlines to properly define chamber borders, even with films of excellent quality. The use of more advanced or partially automated systems also requires an experienced investigator to define ambiguous areas where the diaphragm or ribs cross the heart outline or the aorta passes behind the heart image. Reproducibility often is poor and the task is tedious because of the great number of pictures to be processed. Since the introduction of the digital computer into the cardiovascular laboratory, many aspects of the data processing have been automated. However, the crucial part in the data acquisition and processing is still the definition and subsequent storage of the chamber borders into the digital computer.
In the last decade, many attempts have been made to automate this procedure and to come up with an automated border recognizer. Most methods are based upon computer processing of the digitized left ventricular image, making online and real-time operation impossible.
An electronic system for automatically detecting structures characterized by a change in brightness level when the information is carried by a video signal and processing the picture information has been discussed in a paper by Leo P. de Jong and Cornelis J. Slager, "Automatic Detection of the Left Ventricular Outline in Angiographs Using Television Signal Processing Techniques", IEEE Transactions on Biomedical Engineering, Vol. BME-22, No. 3, May 1975, pp. 230-237. Starting from the available angiographic data regarding the location and brightness level near an already known contour point of the left ventricle for a given video scanning line, a prediction is made of the most probable location and brightness level of the contour point at the next line. The prediction of location, based on the coherence that may be expected between contour points on successive lines, is realized by defining a so-called expectation window for each subsequent contour point and allowing detection only in this expectation window. Brightness information obtained at sample points near the detected contour is used to predict the reference brightness level at which the next contour point may be detected. This system is principally based on analog circuit design with the expectation and sample-and-hold windows determined by time delays generated by monostable multivibrators which does not permit a relatively easy and accurate correlation of video lines and fields. Its other disadvantages include the following. The center of the expectation window on a given line is defined as having the same horizontal position as the last detected border point on the preceding line. The basic limitations of this principle are that only limited excursions from the direction perpendicular to the video scan lines are allowed and the center of the expectation window is generally not a good approximation of the next border point. The position of the sample point, which is determined shortly before the expected new border position, is also defined with respect to this center of the expectation window. This means that the local direction and movement of the left ventricular outline is not taken into account in the positioning of this sample point, resulting in a less accurate determination of the desired video sample and of the calculated reference level. Only two brightness information sample oints are defined for the calculation of the reference level for the next border point to be detected. This does not allow for the determination of the sign of the background slope in the video signal near the actual border; for negative values too high a reference level will be calculated according to the formula for the reference level.